December 6, 2006

A Young, Deep Maria

The limb is like a treasure hunt, with the lucky searcher sometimes delighted with mystery and surprises. True discovery is rare, for mechnical explorers (orbiters) have robotically mapped each hectacre of the lunar surface. But eyeball exploration can easily lead to seeing features for the first time. In this image of the first minutes of of this lunation’s sunset, the farside rim of Smythii basin is illuminated and the nearside scarp is just detectable by its thin shadow. Little trace of Smythii’s ejecta is visible for many younger craters dig into the surface outside the rim, demonstrating the relatively old age of the basin. The floor is obviously two-toned with dark lava at the far right (in association with the floor-fractured Kiess) and the upper left. The latter is some of the Moon’s youngest maria, with an estimated age of 1-2 billion years. The mare surface of Smythii is remarkable in being 5 km lower than the average elevation of the Moon, and in fact the center of the basin is 8 km lower than its highest rim segment. This shallow-looking basin is the same depth as the Orientale basin!